The present invention relates to matrix detectors, and in particular to the real-time identification of bad pixels in such detectors against a non-uniform landscape.
Optical systems based on matrix image sensors or “matrix detectors” are known. Also known are methods of identifying bad or damaged pixels in these matrices. The identifications normally include a factory two-point non-uniformity correction (TPC-NUC). The two-point NUC refers to a test in which a uniform black body image is presented to the detector at two different temperatures. The detector output is measured, and from these measurements one obtains three figures of merit for each picture element (pixel). These three figures of merit are gain (responsivity), level (dark current), and noise. The values of gain, level, and noise should be roughly the same for all the pixels. A pixel that deviates significantly from the others in any of these three figures of merit is considered (tagged) defective (bad) and included in a “bad pixel” table.
Additionally, after the TPC-NUC, there is sometimes performed an on-line, real-time, continuous one-point correction (OPC) in which only the OFFSET is corrected.
This initial tagging of bad or damaged pixels does not assure long-term stable performance of the detector. The matrix may experience during its lifetime deterioration of additional pixels, which need then to be added to the bad pixel table. In other words, the original bad pixel table is unstable in time. Moreover, even when the deviation of bad pixels from “good” pixels is on the order of the temporal noise, the fact that the deviation is constant reflects disadvantageously on the system performance. Detecting such bad pixels includes the steps of a) positioning a uniform black body in front of a static detector, and sampling of a large number of images; b) obtaining gain level and noise figures for each pixel, c) identifying bad pixels that deviate from a mean value by more that a given criterion; and d) adding the identified damaged pixels to the “bad pixel” table. The main problem and disadvantage of this method lies in the need for a uniform black body. Such a body is normally not available under field conditions, and certainly not available during normal operation of the system, for example when the detector is in flight, attached to a body such as a missile.
There is thus a widely recognized need for, and it would be highly advantageous to have, an on-line, real-time, simple, and fast method for identification of bad pixels that appear after the original bad pixel table is formulated.